In European Published Patent Application No. 0 513 427, an optical position-measuring device is described which is suitable for detecting the position of two objects movable relative to each other in at least one measuring direction. To that end, the position-measuring device includes a measuring standard which is joined to one of the two objects. The measuring standard has an incremental graduation extending in the measuring direction, as well as at least one reference marking at a reference position. The reference marking is made up of a structure having a locally changeable graduation period, that is, it is formed of a structure which includes a plurality of different graduation periods. Such structures are also referred to as “chirped” graduation structures or “chirped” gratings. The position-measuring device also includes a scanning unit that is joined to the other of the two objects and has scanning devices which are used to generate at least one displacement-dependent incremental signal, as well as at least one reference signal at a reference position by the optical scanning of the incremental graduation and the reference marking along the measuring path. In so doing, a reference-pulse signal is formed when all signal frequency components, which come about due to the different chirped graduation periods of the measuring standard and of the scanning grating, are superposed in correct phase relation.
In FIG. 8 of European Published Patent Application No. 0 513 427, a special variant of a chirped reference marking formed in this manner is shown, which includes two reference-marking subfields disposed in mirror symmetry with respect to an axis of symmetry. Each of the two reference-marking subfields is made up of a structure extending in the measuring direction and having a locally changeable graduation period.
The position-measuring device described in European Published Patent Application No. 0 513 427 is based on what is referred to as an interferential scanning principle, in which the first grating in the scanning beam path is illuminated by collimated light, that is, is illuminated with a parallel-directed beam of rays. The displacement-dependent scanning signals in the form of the incremental signals and reference signals are obtained from the constructive and destructive superposition of a plurality of partial beams of rays which undergo displacement-dependent phase shifts in the case of the relative movement of the measuring standard and the scanning unit. Highly resolved position information with regard to the relative position of the two objects may be obtained in this manner.
A scanning principle for optical position-measuring devices as an alternative to this is described, for example, in the publication by R. Pettigrew having the title “Analysis of Grating Imaging and its Application to Displacement Metrology” in SPIE Vol. 36, 1st European Congress on Optics Applied to Metrology (1977), pages 325-332. Decisive in this scanning principle is the divergent, i.e., non-collimated illumination of the first grating in the scanning beam path.
German Published Patent Application No. 197 48 802 relates to chirped reference markings in optical position-measuring devices. In addition, in this publication, it is described that such reference markings may also be used in conjunction with the scanning principles indicated above, thus, in conjunction with systems which provide a divergent illumination of the first grating in the scanning beam path. However, neither German Published Patent Application No. 197 48 802 nor European Published Patent Application No. 0 513 427 gives information as to how to specifically design an optical position-measuring device with divergent illumination if chirped reference markings are to be used.